1. Field of the Invention
The present invention relates to a method for producing a glass soot deposit, in particular, a glass preform for use in the fabrication of an optical fiber.
2. Description of the Related Art
In a VAD method which comprises blowing a gaseous glass-forming raw material together with a fuel gas from a combustion burner to form glass soot by a gas phase reaction in a flame and depositing the glass soot on a starting member, environmentally harmful materials such as hydrogen chloride gas are by-products. To trap the harmful materials effectively or eliminate any outside influence on a reaction system, reactions in the VAD method are carried out in an internal space of a closed reactor such as a muffle to deposit the glass soot.
Recently, much research effort has been expended to increase deposition rate, namely synthesis speed of a glass soot deposit per a minute (g/min.) in the VAD method. As an example Japanese Patent Kokai Publication No. 171938/1987 discloses a technique for cooling a muffle tube surface in order to lower the temperature of the glass soot deposit, whereby the deposition efficiency will be increased.
As disclosed in the above Japanese Kokai Publication, the conventional technique for cooling the muffle tube surface decreases the surface temperature to about 20.degree. C.
However, since an oxyhydrogen flame is generated in the muffle tube, the dew point therein is higher than that in the external atmosphere. Therefore, when the muffle tube surface is cooled down to about 20.degree. C., dew condensation occurs in the muffle tube. Since hydrogen chloride gas is generated in the muffle tube, it reacts with water generated by dew condensation to form hydrochloric acid, which corrodes the metal muffle tube. Therefore, the life of the muffle tube is shortened and the corroded metal contaminates the glass soot deposit (preform). Ultimately, when an optical fiber is fabricated from such a contaminated preform, the fiber has increased transmission loss.